1. Field of the Invention
The invention relates to a semiconductor switching device for switching at high frequencies, especially to a compound semiconductor switching device comprising two switches connected to each other.
2. Description of the Related Art
Mobile communication devices such as mobile telephones often utilize microwaves in the GHz range, and commonly need switching devices for high frequency signals which are used in switching circuits for changing antennas and switching circuits for transmitting and receiving such signals. A typical example of such a switching device can be found in Japanese Laid-Open Patent Application No. Hei 9-181642. Such a device often uses a field-effect transistor (called FET hereinafter) made on a gallium arsenide (GaAs) substrate, as this material is suitable for use at high frequencies, and developments have been made in forming a monolithic microwave integrated circuit (MMIC) by integrating the aforementioned switching circuits.
FIG. 1A is a cross-sectional view of a conventional GaAs FET. The GaAs substrate 1 is initially without doping, and has beneath its surface an n-type channel region (or a channel layer) 2 formed by doping with n-type dopants. A gate electrode 3 is placed on the surface of the channel region 2 forming a Schottky contact, and a source electrode 4 and drain electrode 5 are placed on both sides of the gate electrode 3 forming ohmic contacts to the surface of the channel region 2. In this transistor configuration, a voltage applied to the gate electrode 3 creates a depletion layer within the channel region 2 beneath the gate electrode 3 and thus controls the channel current between the source electrode 4 and the drain electrode 5.
FIG. 1B shows the basic circuit configuration of a conventional compound semiconductor switching device called a SPDT (Signal Pole Double Throw), using GaAs FETs. One of the source electrode and the drain electrode of each FET (FET1 and FET2) is connected to a common output electrode OUT. Another of the source electrode and the drain electrode of each FET (FET1 and FET2) is connected to respective input terminal pads (IN1 and IN2). The gates of FET1 and FET2 are connected to the control terminal pads Ctl-1, Ctl-2 through resistors R1, R2, respectively. A complementary signal is applied to the two control terminal pads, Ctl-1, Ctl-2. When a high level signal is applied to the control terminal pad of one of the FETs, the FET changes to an on-state, and a signal passes through the FET and reaches the common output terminal pad OUT. The role of the resistors R1 and R2 is to prevent the leaking of the high frequency signals through the gate electrodes to the DC voltages applied to the control terminal pads (Ctl-1 and Ctl-2), which are substantially grounded at high frequency.
When a mobile communication device can use two kinds of communication methods, for example, CDMA and GPS, the switching device of the communication device needs to alternate between the two communication methods. However, in many instances, the signals of these communication methods are provided as two balanced signals, which have opposite phases (180 degree difference) from one another. Accordingly, two separate switches are required for this kind of device and occupy a large space corresponding to the two switches and their wiring in the device.